Surface roughness and integrity

The video introduces the concepts of surface finish and surface integrity, defining a surface as the border between a workpiece and its environment. Surface integrity encompasses the state and attributes of a machined surface, relating to its functional performance. It includes surface topography (finish) and microstructural properties, mechanical properties, and residual stress of the subsurface layer. Performance characteristics sensitive to surface integrity include fatigue strength, fracture strength, corrosion rate, tribological behavior, and dimensional accuracy.

Surface science involves absorption, surface physics, topography, and chemistry. Surface engineering focuses on forming, investigating, utilizing, and designing surface layers to achieve desired properties like improved corrosion, oxidation, wear resistance, reduced friction, enhanced mechanical properties (fatigue life, hardness), and improved electrical, thermal, biological, and aesthetic aspects.

The video presents examples of surface finish in longitudinal turning for AISI 1045 material. It demonstrates how cutting speed influences surface roughness; lower cutting speeds (e.g., 12 m/min) result in higher surface roughness, while higher cutting speeds (e.g., 250 m/min) produce lower surface roughness. Typical surface finishes for various basic machining operations like planing, drilling, and turning are also discussed.

Surface technology involves visual and tactile contact, revealing roughness, waviness, reflectivity, scratches, cracks, and discolorations. It covers integrity, structure, texture, and roughness. Tribology, a related field, deals with friction, wear, and lubrication. Surface treatments include burnishing, hardening, deposition, implantation, coatings, and cleaning. Surface texture and defects significantly influence the integrity of parts and dies.

Surface texture describes the characteristic features of a surface. The most common measurable quantities are roughness (closely spaced irregular deviations) and waviness (recurrent deviations forming repeating crests and valleys). Roughness is defined by height, width, and distance, while waviness is measured by the distance between adjacent crests or width. Key parameters include roughness width, height, waviness height, and direction of finish pattern.

RA (arithmetic average roughness or centerline average) is the most popular parameter for process and product quality control due to its ease of definition and measurement. However, it's insensitive to small profile variations and doesn't distinguish between peaks and valleys. RT (maximum peak to valley height) is sensitive to deviations and scratches. RZ (average peak to valley height) is less representative than RT. RQ (root mean square roughness) is more sensitive to peaks and valleys than RA. RP and RV define the highest peak and deepest valley, respectively.

Different formulas are presented for calculating RA (arithmetic mean value) and RQ (root mean square roughness), involving absolute values and the number of readings. Surface roughness is typically measured using a surface profilometer with a diamond stylus that travels along a straight line over the surface, recording its profile. Examples of resulting roughness values for processes like lapping, grinding, and turning are provided.

The video explores the theoretical forms of surface roughness in turning and milling, including formulas for maximum roughness (RT). It demonstrates how cutting conditions, such as feed rate and cutting speed, affect surface roughness (RA). Higher feed rates generally lead to higher RA values. Conversely, surface roughness increases with a decrease in cutting speed. The depth of cut typically has no significant influence on surface roughness in the finishing range.

Surface finish describes the geometric features, while surface integrity pertains to material properties, influencing performance characteristics like fatigue strength, corrosion resistance, and service life. Surface integrity encompasses topographical, physical, chemical, mechanical, and metallurgical properties. These properties, both superficial and in-depth, affect performance and include texture, profile, fatigue, corrosion, wear resistance, adhesion, diffusion, and other optical, thermal, and biological attributes.

A simplified checklist for surface integrity (SI) includes determining working conditions, identifying material requirements, analyzing failure modes, considering one-piece construction or substrate material groups, and selecting appropriate manufacturing and surfacing processes. Surface defects are caused by material defects, machining processes, and improper control of parameters. Common defects include cracks, craters, pits, heat-affected zones, recast layers, laps, metallurgical transformations, and residual stress. Solutions involve rough machining to remove bulk material and then finish machining with lower feed rates, depths of cut, and appropriate cutting speeds.

Machinability is defined by four factors: surface finish and integrity, tool life, force and power required, and chip control difficulty. Good machinability means good surface finish and integrity, long tool life, and low force/power. The video discusses the machinability of various metals: aluminum and silicon in steel are harmful; stainless steels are difficult to machine (except ferritic types); aluminum is easy; beryllium needs controlled environments; cobalt-based alloys require sharp, abrasion-resistant tools; copper can be difficult due to built-up edge; magnesium is easy; titanium has poor thermal conductivity; and tungsten is brittle and abrasive. Machining thermoplastics and composite materials also presents unique challenges depending on their properties.